Microcellular heterocyclic polymer structures

ABSTRACT

Highly useful novel microcellular polymeric structures, especially films and fibers, are prepared from certain solid polymers. Aromatic polysulfones, polyimides, polyhydantoins, polyamides and polyparabanic acid are the preferred ones for the novel structures of the invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 866,444, filed Jan. 3,1978, now abandoned which is a continuation-in-part of copendingapplication Ser. No. 833,535, filed Sept. 15, 1977, now abandoned whichin turn is a Rule 60 Continuation of Ser. No. 356,924, filed May 3,1973.

Some of the preferred resins employed in accordance with the inventionare disclosed in U.S. Pat. Nos. 3,591,562; 3,547,897; 3,661,859;3,684,773; 3,637,843; and 3,635,905.

BACKGROUND OF THE INVENTION

The technique of casting polymeric articles is old and well established.In general, there are two approaches to casting. One of these is castingeither a monomeric or partially polymerized syrup into a mold or shapeand conveying this into an oven or autoclave in order to finishpolymerizing the article with a temperature treatment. The other generaltype of casting involves solution casting which is also a long utilizedtechnique for producing plastic film and sheet materials.

The general technique of solution casting involves forming a solution ofthe film-forming polymer into a suitable solvent, casting the resultingsolution on a suitable substrate, evaporating the solvent and windingthe resultant film on rolls.

Usually solvent recovery systems are employed in order to recover thesolvent and minimize the loss of an expensive process component.

Solution cast opaque films have been conventionally prepared by addingpigments, fillers, flame retardants and solubilizers to a solution ofthe film-forming material, which pigment acts as an opacifying agent.Without such an agent such film would be colorless or transparent.Opacifying agents often embrittle the film.

Various processes have been described in the art for preparing opaquefilms which rely for opacity upon the presence of a large number ofvoids in the film. Such films may be prepared by depositing a film froman emulsion, i.e., either an oil-in-water or a water-in-oil emulsion.

When a water-in-oil emulsion is used--i.e., one in which minute dropletsof water are dispersed in a continuous phase of a film formingmaterial--the emulsion is deposited as a coating and the organic solventwhich comprises the continuous phase of the emulsion is evaporatedtherefrom. This causes gelation of the film-forming material andentrapment of the dispersed water droplets. The water is then evaporatedleaving microscopic voids throughout the film structure.

Still another technique for obtaining a porous, opaque, nonpigmentedfilm is set forth in U.S. Pat. No. 3,031,328. Basically, this processcontemplates preparing a solution of a thermoplastic polymer material ina mixture of a volatile organic solvent and a volatile non-solventliquid which has an evaporation rate substantially less than that of thesolvent. The clear homogeneous solution is then coated on a suitablebacking material and dried by evaporation to produce an opaque blushedfilm which is adapted to be rendered locally transparent by heat orpressure. These films are useful as recording films.

Other techniques for forming opaque, porous, nonpigmented, microporousthermosetting films are set forth in U.S. Pat. No. 3,655,591.

Nevertheless, in spite of the above, the art has never appreciated theunique articles which result when a specific type of polymer is cast ina certain manner to produce an essentially non-porous, non-foammicrocellular structure which has unique and unusual properties and isincidentally opaque. The art has concentrated on techniques wherein theopaqueness is the sine qua non of the structure and the other propertiesare not of significance.

SUMMARY OF THE INVENTION

Unique microcellular, non-porous, non-foam polymeric articles such asfilms and fibers are prepared by a novel solvent/non-solvent castingtechnique.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is known that films, fibers and other structures can be made out ofthe solvent cast polymers such as those described in U.S. Pat. No.3,661,859, which disclosure is incorporated herein in its entirety byreference. Those particular polymers are referred to as1,3-imidazolidene-2,4,5-trione-1,3-diyl. The repeating heterocyclic ringstructure of these polymers is shown as follows: ##STR1##

Other preferred polymers are aromatic polyamides, aromatic polysulfones,and polyhydantoins which have been described in the art. See, forinstance, Netherlands Pat. No. 6809916, Belgium Pat. No. 723,772, GermanPat. Nos. 1,807,742; 1,805,955; 1,812,002; 1,812,003; and 1,905,367.Polyimides are well known and are described in such publications asBritish Pat. No. 1,240,665, U.S. Pat. No. 3,486,934, U.S. Pat. No.3,536,666, French Pat. No. 1,488,924, French Pat. No. 1,549,101, RussianPat. No. 218,424, German Pat. No. 1,301,114, Netherlands Pat. No.7,001,648 and the like.

The detailed preparation of these polymers and solutions of thesepolymers in suitable solvents are set forth in the above-recited patentsand others also in the art, and therefore need not be repeated hereexcept as is necessary to understand the invention.

The preferred microcellular structure from the polymers of the inventionare characterized by high temperature thermal stability, organic solventresistance, relative high tensile modulus, tensile strength and ultimateelongations with low shrinkage at high temperatures and are slow smokeformers when ignited.

Non-microcellular film from the preferred polymers have relatively highdielectric strengths. These properties have been found to offeroutstanding advantages when used as films in flexible circuitry, for usein auto air bag circuits, light monitoring circuits, and telephonecircuits because of their ability to be soldered. They also are useablefor fibers, where high tenacity and modulus are required.

However, in these applications the structure has a relatively high costper unit of weight. It would be desirable to have a structural articlepossessing essentially the outstanding properties of the above describednon-cellular film so that it can be used for the applications listedabove, but it would be less dense. If products of low density and stillsuperior properties could be obtained, it would mean that a novel newstructure of outstanding cost-performance utility would exist.

It has been discovered and forms the fundamental substance of theinvention that such relatively low density microcellular structures canbe prepared and are novel themselves. These films are very thin and areessentially non-porous, i.e., the microcells are--closed-celled. Thetechnique of preparing them forms a portion of this invention.

If the advantages delineated above for the lower density material wereall that the material contributed, its existence would be welcomed andits utility would be considered outstanding. Notwithstanding theoutstanding utility of the lower density material, it has beendiscovered that the material has additional unique properties of its ownwhich make it extremely valuable in addition to those propertiesenumerated above.

The preferred polymer species of the invention are polymers of1,3-imidazolidene-2,4,5-trione, i.e. polyparabanic acid, herein referredto as PPA, andpoly(imino-1,3-phenyleneiminocarbonyl-1,3-phenylenecarboxyl),hereinafter referred to as IPP. The particular conditions, reagents anduses are especially well suited for the PPA or IPP polymers andstructures resulting therefrom. Nevertheless, it must be emphasized thatother polymers of this invention can be handled in an analogous mannerto make structures which have some similar properties. These latterinclude the soluble polyamide-imide, polyimides, polysulfones,polyamides, and various soluble polyhydantoins.

In general, the polymers of the invention will be comprised ofsufficient repeating units to be solids at room temperature. Therepeating unit can contain heterocyclic rings.

The heterocyclic ring will be 5-membered and will contain carbon, andnitrogen linkages wherein at least two of the carbons will be in theform of carbonyl groups, i.e. ##STR2## which are separated by a nitrogenatom.

Examples of heterocyclic rings which fall in this case are: ##STR3##

Other preferred polymers have repeating units as follows: ##STR4##

Wherein Z is a number from 20 to 1,000, preferably 50 to 200.

Although casting in general is a relatively well-known process, for eachpolymer and solvent system there are unique problems brought about bythe particular solvents which must be used and the properties of polymeritself. Very generally, PPA's are soluble in moderate hydrogen bondingdipolar, aprotic solvents. This presents a practical problem in casting,since solvents which are available at a reasonable cost have relativelyhigh boiling points and are of low volatility, except at relatively hightemperatures. The effect of these parameters is that when PPA is castinto even relatively thin structures, a film, for instace, it isrelatively difficult to remove the last small amounts of solvent fromthe structure, e.g. film.

For instance, dimethylformamide (DMF) is considered to be one of thebest solvents for working with PPA solution formulations. It boils at156° C. and its excellent solvating effect results in the fastdissolution of PPA along with the formation of low viscosity solutions.

Nevertheless, this combination of low volatility and high solvation,which characterize a good solvent makes the removal of the last amountsof solvent from even thin structures such as films very difficult.Therefore, film casting processes must be conducted with extremely highdrying temperatures in order to get good solvent removal at reasonableproduction rates.

In accordance with this invention low density nonporous, i.e., themicrocells are closed-celled, microcellular film structures, e.g. PPA,aromatic polyamides and others listed above are prepared by firstsolvent casting of film. Prior to complete drying one precipitates thefilm by contacting the film with an antisolvent which is also known tothe practitioners in the art as "non-solvent", such as water. A basicrequirement for the antisolvent is that it be miscible with the solventin the polymer solution.

More specifically the inventive process involves the steps of:

(a) preparing a casting solution of the polymer;

(b) casting a wet film onto a surface or extruding a fiber;

(c) partially drying the cast film or fiber;

(d) contacting the wet film with an antisolvent such as water; and

(e) removing solvent and antisolvent by completely drying the nownon-porous, microcellular article.

In order to obtain a film with very low density, one can eliminate steps(c) and for step (d) expose the film to an atmosphere of high humidity.

Additives, such as flame retardants, oxidation inhibitors, plasticizers,etc. should be dissolved in the solvent with the resin prior to casting.

The solvents which can be employed in accordance with this invention aremoderate hydrogen bonding dipolar, aprotic solvents. These solvents havebeen described in U.S. Pat. No. 3,661,859. The preferred solvents areN,N-dimethylformamide, N-methylpyrrolidone, N,N-dimethylacetamide anddimethyl sulfoxide.

The anti-solvents as mentioned above must be miscible with the solvent.Typical of the anti-solvents are water, aliphatic alcohols such asmethanol, ethanol, propanol, butanol and the like; aliphatic ethers suchas methyl ether, ethyl ether, methyl ethyl ether, propyl ether, methylpropyl ether, ethyl propyl ether and the like; and aliphatic ketonessuch as acetone, di-ethyl ketone, methyl ethyl ketone and the like. Thepreferred anti-solvent is water.

The concentration of the resin in the solution should be such as to notproduce a viscosity which would make the solution too difficult tohandle. Typically the suitable viscosities can be determined by simpleexperimentation.

Generally for ease of operation, the concentration of the resin in thecasting solution may be such that the Brooksfield viscosity at 25° C. isbetween about 80 and about 800 poises. Desirably, the viscosity for thegreatest ease of operation can be between about 200 and 300 poises.

Prior to casting it is desirable to filter the casting solution so as toremove any trash and gel particles.

In general, there are two methods according to the invention than can beused at step (d) described above to form the novel cellular articles ofthe invention. These are:

(a) Method 1--The wet film or fiber, is exposed to an atmosphere of highwater humidity, followed by a direct water washing, followed by drying.As is true of all of the techniques, the thickness and shape of thestructure is controlled by its original cast or extruded thickness andshape and solids content. Precipitating the structure in a high humidityenvironment rather than initial direct water contact is important. Thereason is that too rapid precipitation and solvent removal will causewrinkling of the structure, which is very undesirable.

(b) Method 2--The wet structure, e.g. film or fiber, is obtained bysolvent casting or extrusion, then it is partially dried to a greater orlesser extent. This serves two purposes; prevents wrinkling andincreases the density of the structure. Then it is water washed anddried completely. The density will vary according to the amount ofsolvent removed in the initial drying step.

Method 1 gives a density of about 0.45 g. per cubic centimeter, Method 2gives a density varying from about 0.3 to 1.5 and preferably 0.3 to 1.2g per cubic centimeter. When operating in the solution casting mode, thefollowing considerations will be pertinent.

Density is largely dependent on the weight fraction of polymer in thewet film at the instant precipitation occurs. Casting solutions of PPAranging above 30 weight percent resin are difficultly handled inconventional solvent casting equipment due to their very high viscosity.The high viscosity solution can nevertheless, be readily obtainedthrough extrusion through an appropriately shaped die.

The Method 1 technique contemplates the use of the most viscous solutionthat can be handled, i.e. 20 to 50 weight percent PPA depending uponmolecular weight of the polymer.

Method 2 permits the use of a more dilute solution with its concomitanteasier handling advantages, and relies upon the evaporation of moresolvent from the film after it is cast and prior to first precipitation.This allows a wider range of densities than can be obtained by Method 1.

The practical limit which sets the maximum density which can be obtainedin Method 2 is the minimum amount of solvent which must remain in thepolymer in order for precipitation to occur when the cast polymersolvent structure is contacted with water or other antisolvent.

The minimum density of Method 2 is limited by the maximum amount ofsolvent that can be left in the wet film at the precipitation step,which will not cause surface wrinkles on the film or fiber surface. Thiswill vary depending on the choice of anti-solvent as can be simplydetermined.

The range of densities can be further increased by (a) calendering theresulting cellular film, (b) orienting the film and the fibers elongateand reduce the volume of the cellular portions, or (c) the use ofdifferent mechanical equipment designed to handle the extremely viscouspolymer solutions, for example slot extruders. That latter approachwould increase the density of the microcellular material approximatelyproportionately to the amount of solvent reduction in the originalpolymer solvent solution. Thus when solution extrusion equipment isused, much higher polymer solvent contents can be handled as compared tothe casting methods described above.

In accordance with this invention the non-porous, i.e., the microcellsare closed-celled microcellular structures can be prepared as a freefilm, a permanent coating on a surface or as an impregnating substance.

The free film is prepared by laying down a layer of casting solutiononto the desired flat surface which conveys the wet layer sequentiallyinto a drying zone so as to partially dry the film, a water zone (bath,vapor or spray) and to a final drying zone. Suitable surfaces which maybe employed are metal, which has been polished or embossed, chromeplated metals, release paper and others known in the casting art. Whenneeded, a release agent can be included in the casting solution tofacilitate removing the finished film from the casting surface. Asindicated above, so as to obtain a very low density film, the firstdrying step can be eliminated.

Suitable equipment for laying down the wet film of casting solutions arecasting boxes, reverse roll coaters and pressured extrusion dies. Thechoice depends upon the thickness of wet film to be laid down and theviscosity of the casting solution. With casting solutions having aBrookfield viscosity less than 200 poises one can employ a reverse rollcoater. Intermediate viscosities and wet film thickness are handled bestby casting boxes. The typical ranges are Brookfield viscosities of 100to 300 poises at wet film thickness of 10 to 30 mils. Very highviscosities, 300 to 1000 poises require extrusion die equipment. Othermethods of laying down film would be known to those practitioners in theart.

The wet film exposed to the first drying zone will have the initialcomposition of the casting solution. After partial drying or in theabsence of the partial drying step the solvent content of the filmshould preferably be between about 20% to 50% by weight. Nevertheless,greater or lesser percentages may be employed. However, if less than 20%solvent is present, the desired cellular structure will be formed in thewater zone slowly and the cellular structure will not as perfectlyformed. If the residual solvent is greater than 50%, the rate of waterabsorption will be too rapid causing the film to be deformed. In betweena well formed cellular structure will be produced with little or no filmdeformation.

The water zone may consist of water vapor, water spray, a water bath, orany combination of these as long as water is rapidly absorbed by thefilm. In addition to water being absorbed into the film, solvent isextracted from the film such that the solvent content of themicrocellular film should be less than 10%. During the final dryingstep, the microcellular film will tend to melt with loss ofmicrocellular structure if the solvent content is much greater than 10%.

As the film is partially dried and as it is contacted with water thefilm thickness and cell diameter will decrease due to removal ofsolvent.

The film is dried in a final drying zone which preferably is a stagedzoned oven having temperature gradient of from 175° to 270° C. The finalsolvent content is usually <3000 ppm. A further reduction in filmthickness and cell diameter occurs during the final drying.

The properties of the finished microcellular film are basicallydetermined by (1) film thickness, (2) film density, and (3) celldiameter. It is apparent that final film thickness is substantially lessthan that of the initially cast wet film since as indicated above thethickness is reduced in each step of the casting operation. The largestreduction must occur in the first drying zone since from 40% to 75% ofthe solvent is desirably removed. Thus, if a particular thickness offinished microcellular film is to be obtained, the initially cast wetfilm must be from 2 to 3 times this thickness. The upper practical limitof thickness of the finished microcellular film of this invention isabout 20 mils. This is to be contrasted with foams which are formed byan expansion process. Thus, the thickness of the foamed article and thecell diameter increase during the foaming process. This imposes apractical lower limit of greater than 20 mils thickness for foamedarticles. The lower practical thickness for these microcellular film isabout 1 mil which is far below that attainable by foaming. Hence, as apracticable matter the films of this invention range from about 1 mil toabout 20 mils in thickness.

Film density is governed by the volume fraction of the microcells. Forexample, if the volume fraction is 0.50, then the density of themicrocellular film will be 50% of that of dense film from the sameresin. The density of microcellular film has a practical lower limit ofabout 30% of the dense film value. The upper limit is about 90% of thedense film value. Foams by contrast are usually less than 30% of thecorresponding dense article. The range of cell diameters usefullyemployed in accordance with this invention can be from about 0.1 toabout 10 microns. Although cell sizes can be greater or lesser, for mostuseful applications, the smallest cell diameter is preferred such asfrom about 0.1 to about 5 microns, although there are some exceptions tothis rule. In any event, the mechanical strength, compressability andtoughness are increased the smaller the microcells for any given filmdensity. By contrast, foamed articles usually contain cells havinggreater than 10 microns diameter and their mechanical strength andtoughness are low. The shape and distribution of the microcells are alsoimportant. In general, microcellular articles, however they may be made,have structures made up of either open or closed cells. All of the filmof this invention contain predominantly isolated spherical closed cellswhich have such a uniform distribution that only occassionally do twocells impinge on one another. By virtue of the essentially discretespherical closed cells, the transmission of gases, vapors and liquidsare so slow the film that it can be considered to be impermeable whencompared to semi-permeable microporous membranes and foams.

Cellular film made from PPA solutions in DMF were prepared on equipmentwhich is normally used to make porous cellulose acetate film forelectrophoresis applications. The equipment consisted of a casting boxapplicator, a sixty-foot continuous stainless steel belt, and fourchambers equipped to control humidity, temperature and the rate of airflow.

Provisions were also incorporated to spray water onto the movingcontinuous belt for the purposes of initial precipitation and forwashing the solvent from the film.

The first technique used was that described for Method 1. And thepolymer was PPA in 20% concentration in DMF. The humidity was controlledat 90°-95° in the first zone. The film initially precipitated due toabsorption of water vapor. Additional precipitation and solvent removalwas affected by direct immersion in a water bath followed by drying.

Under these conditions of high humidity the wet film absorbs water vaporrapidly due to the hydroscopic nature of the DMF solvent but much slowerthan if directly immersed in water. Films having a uniform cellularstructure were obtained having densities of about 0.45 g. per cubiccentimeter.

The belt speed varied from about 0.50 feet per minute to about 2.5 feetper minute. The temperature was about 100° F. The air rate was about 900to 1,300 cu. ft. per minute. The thickness of the wet film varied fromabout 8 mils to about 20 mils. The total time in the oven ranged fromabout 6 minutes to about 15 minutes. Generally time periods above 10minutes and less than 20 minutes appeared to be satisfactory.

Subsequent work has been done to produce cellular films which haverelatively high densities, i.e. up to 1.1 gm/cc.

For this, the above-described equipment was modified somewhat to permitthe use of a Method 2 type approach. It required the addition of heatersto the first chamber in order to provide for some gradual initialsolvent removal, which is a step required to control the increase in thedensity of the film and to prevent wrinkling. Equipment to spray wateronto the moving stainless steel belt was installed in the second chamberto precipitate the film. Water sprays in the third and fourth chamberswere provided in order to wash out additional solvent, e.g.N,N-dimethylformamide (DMF) prior to stripping the film off the belt andsubsequent drying.

Passage of the solvent loaded film into an irregular water interfaceproduced a non-uniform surface on the precipitated film. Therefore, anair knife was installed which directed an air flow downward onto thesurface of the belt and provided a relatively uniform water interfacefor the wet film to pass into.

Although useful microcellular articles in particular foams andsemi-permeable membranes having thickness >0.020" have previously beenknown, such structures have not been found to have utility as a highperformance engineering material in thinner gauges. By contrast, theytend to be of relatively low strength. For the first time, themicrocellular film of this invention provide materials in thin gaugeswhich combine the properties necessary for high performance engineeringmaterials with some of the desirable characteristics of light weightmicrocellular materials. Some of the more important properties areillustrated in Table I where microcellularpoly[1,4-phenylenemethylene-1,4-phenylene-1,3-(imidazolidine-2,4,5-trione)]which is designated in Chemical Abstracts aspoly[(2,4,5-trioxo-1,3-imidazolidinediyl)-1,4-phenylmethylene-1,4-phenylene] hereinafter referred to as PPA-M is compared toseveral commercial foams and dense film made from engineering plastics.The aromatic polyamides, polyimides, polyhydantoins, polysulfones andpolyamide-imides of this invention have properties similar to the PPAfilms.

                                      TABLE I                                     __________________________________________________________________________                     Commercial Foams                                                        Cellular                                                                            (Flexible Sheets)                                                                          Non-Cellular Film                                          PPA-M Poly   Poly        Poly   Poly                                          Film  Urethane(a)                                                                          Styrene(a)                                                                          PPA-M Carbonate(a)                                                                         Sulfone(a)                         __________________________________________________________________________    Density (gm/cc)                                                                          0.5-1.0                                                                             0.02-0.04                                                                            0.16  1.35  1.2    1.25                               Tensile Strength (psi)                                                                   5000-10,000                                                                         8-25   600-1000                                                                            14,000                                                                              8,600  9,500                              Elongation (%)                                                                           100-150                                                                             (b)    (b)   50-100                                                                              85-105 64-100                             Tearing Strength                                                              (gm/mil)   8-15  (b)    (b)   6-10  20-25  9-12                               Dielectric Constant                                                           (10.sup.3 cps)                                                                           1.5-2.8                                                                             1.0-1.5                                                                              1.28  3.4   3.0    3.1                                Dielectric Strength                                                           (KV/mil)   0.5-3.0                                                                             <0.5   <0.5  3-5   1.5    2.5-7.5                            Resistance to Heat                                                            (°F.)                                                                             525° F.                                                                      275° F.                                                                       175° F.                                                                      525° F.                                                                      270° F.                                                                       350° F.                     __________________________________________________________________________     (a)Properties from "Modern Plastics Encyclopedia" 1974-75.                    (b)Property not listed.                                                  

The above data show that microcellular PPA-M film are competitive with awidely used non-cellular film from engineering thermoplastics in tensilestrength, % elongation, tearing strengths and dielectric strengths. Onthe other side, the dielectric constant of the microcellular film ofthis invention is comparable to the very structurally weak foamedsheetings. The very low dielectric constant is of great utility whenused in electrical insulation for both power and signal transmission.Thus, the microcellular film of this invention provide a dielectricinsulation which has a desirably low dielectric constant and at the sametime high dielectric and mechanical strengths. Semi-permeable membranesalthough not used for structural or insulation application would be evenless useful than foam in these applications due to their high porosityand low strengths.

Although it is predictable that mechanical properties such as modulusand tensile strength will decrease with decreasing density, it was foundthat these mechanical properties were not sufficiently diminished toseriously affect the utility of the cellular article for manyapplications. Moreover, in the case of the film, the propogating tearstrength was better than that of the dense film.

The dielectric constant will decrease with decreasing density, andtherefore, the dielectric constant for the cellular products are lowerthan that for the dense film products. This makes the cellular film moreattractive for use as insulation, e.g. for microwave circuitry,especially where transmission is to be over relatively long distances.In an analogous fashion, the lower thermal conductivity makes thesestructures desirable for thermal insulation.

As is the case of the dense film, the cellular film also withstandscommercial solder bath temperatures, i.e. 500° F. vs. <300° F. forfoamed sheetings.

One important and highly advantageous property of the cellular film, asopposed to the dense film is that copper circuits can be electroplateddirectly onto the cellular film, with much higher peel strengths for theelectroplated copper on the cellular film than on the dense film.

For example, peel strengths for copper electrodeposited on the densefilm were in the range of about 2.5 to 3.0 pounds per inch. But peelstrengths on copper electrodeposited onto the microcellular film were inthe range of about 8 pounds per inch.

This is an extremely important aspect of the cellular film which givesit an outstanding advantage, taken in combination with its otherproperties, over dense film.

Not only are the adhesion values exceedingly high for the electroplatedcopper circuit on cellular film, and also laminates prepared withadhesives but the use of the cellular film permits the omission of abothersome process step. Thus, before copper laminated onto plasticfilms which normally contain small quantities of absorbed water can besoldered, the unit must be dried to remove absorbed water. If it is not,the absorbed water tends to be driven from the film during the solderingoperation, because of heating of the composite unit. This rapidgeneration of steam causes the copper to be delaminated from the filmsubstrate.

When the cellular film of the invention is utilized, the copper does notdelaminate. It is theorized that this is due to the fact that there arenumerous microcellular voids into which the water can expand rather thanescaping through the surfaces. Therefore, delamination is effectivelyprevented. This is an exceedingly useful property.

These films are much more flexible than dense film of the samethickness, which is an advantage for thick multilayer structures.

Another important feature of the structures of the invention involvesselective surface etching by strong bases or acids. This removes thefilm covering the microcells, either completely or in any patterndesired. The exposed microcells, can then be electro or chemicallyplated with far better adhesion. In fact, grooves can be etched into thesurface into which conductive metals can be deposited with excellentadhesion to the exposed microcells and with excellent separation andinsulation from adjacent conductor-filled grooves.

All in all, cellular film, because of its combination of properties andits relatively low cost is an ideal material for flexible circuits andflat conductor cables.

The preferred structures produced by the practice of this invention arecharacterized by the presence therein of a large number of discreteclosed cells. Substantially all of these cells or voids are less than 25microns, and preferably less than 2 microns, in size. Most preferablythe cell size is less than 5 microns. The average cell size and cellsize distribution is governed by the conditions under which thestructures are made, e.g., temperature solvent, anti-solvent, polymersolids content of casting solutions, etc. The range obtainable is fromabout 0.1 to 25 microns.

Unless some color-forming material has been included in the composition,such as a soluble dye, the preferred films of this invention are opaqueand off white. Colored films may be obtained by incorporating smallamounts of dyes.

A film having an apparent thickness of, for example, 10 mils will have areal solid thickness which is equal to the sum of the thickness of eachwall between the discrete cells lying along a path perpendicular to theoutermost planar surface of the film which may be, for example, no morethan 3 mils. Thus, the film is of sufficient apparent thickness toprovide the required amount of strength.

Furthermore, the diffusion per unit of time of a vapor or a liquidthrough a unit area of the films of this invention is far smaller thansemi-permeable membranes.

The compositions of this invention are particularly useful whenprecipitated onto fabrics made from fiber glass, resinous yarns,vegetable or cellulosic yarns and cords. When these fibers or cords arecoated with the structures of this invention, an opaque or white fabricis obtained without the addition of pigments as needed in the fabricheretofore employed. These coated fabrics have very desirableflexibility. The fact that pigments such as TiO₂ are not needed toobtain whiteness in fibrous fabrics is quite significant since this hasbeen a problem in the art due to the adverse effects these pigments haveon the resulting fabrics. For example, it is known that pigments such asTiO₂ weaken the tensile strength of the fabric.

The fibers may be coated with the compositions of this invention byeither of the first two methods described above. One method found to besuitable is to dip the fibers into a solution which contains resin,solvent and non-solvent in amounts indicated hereinabove. Uponprecipitation a fabric having the desired whiteness and softness isobtained without the addition of pigments such as TiO₂.

Although the above discussion has been made with reference to films asdiscrete articles, it is to be noted that films in terms of surfacecoatings with unique and important properties and which are bonded to asubstrate can also be produced according to the technique of theinvention.

The structures of this invention may be formed as surface coating filmsby either of the techniques described above. Thus, they may be appliedby extrusion, brushing, spraying, dipping, roller coating, or knifecoating followed by precipitation and drying.

The compositions of this invention are particularly useful when employedin spray applications.

As already indicated compositions of this invention may be applied asfilms to various types of surfaces or substrates. These surfaces may beof the type whereby the film is to be removed by a suitable method or ofthe type where it is adhered to the final substrate such as the metal ofan automobile. Among the more suitable surfaces which may be coated withthe cellular structures of this invention are steel, treated steel,galvanized steel, concrete, glass, fabrics, fiber glass, wood, plasterboard, aluminum, treated aluminum, lead, copper and plastics. The mostpreferred surfaces are metals such as treated steel and treatedaluminum.

Films formed from the compositions of this invention may be air dried,vacuum dried or bake dried at elevated temperatures.

Although considerable emphasis has been placed on cellular filmformation and applications, it is an important feature of this inventionthat cellular fibers of high strength can be produced utilizing thetechnique of the invention.

Fibers made by the conventional wet spinning techniques of the art arenever left in cellular form, but are remelted and oriented in order toeliminate the cellular structure which gives rise to fibers having lowmodulus. In this invention the polymers used have such a high modulusthat the microcellular fibers can be used with onlymoderate-orientation. Ordinarily orientation is used to improve fiberstrength.

This gives rise to a microcellular fiber which can accept dyes readily.Moreover, the fiber has the capacity to absorb moisture. Thus, it willbe comfortable in contact with the human body. The capability ofabsorbing moisture is often the difference between synthetic fabricswhich may feel clammy and natural fabrics such as cotton, the latterbeing much more comfortable because of its water absorptive capacity.Permanent press fabrics can be made due to high softening temperaturesof these novel fibers.

The microcellular films, fibers and other structures can also beelectrocoated with various metals such as copper, aluminum and the likein order to form thin conductive coatings with a minimum of coatingmetal.

Electrocoated structures can be used in a wide variety of decorative andutilitarian applications. These involve automotive trim, under-the-hooduses, and radiation shields.

Electrodeposition and chemical metalizing can also be used to coatcatalytic metals such as paladium, platinum, nickel, and the like withinthe interstices of the structure so that it can be used to form anextremely high surface area, artificial surface for conducting catalyticreactions at relatively high temperatures.

The cellular structures of the invention are also highly useful forspecialty applications where highly tenacious painted surfaces arerequired.

The invention is further illustrated by the following examples.

EXAMPLE 1

Utilizing the technique and apparatus described above, a series of runswas carried out in the apparatus. A casting solution was prepared bycombining 19.7 weight percent PPA, 79 weight percent DMF and 1.3 weightpercent of octabromobiphenyl (which is an excellent flame retardant forPPA film).

All PPA utilized in these examples was prepared from the reaction of HCNwith diphenyl methane diisocyanate.

Three rolls of wet film were cast onto a moving belt from the preparedsolution and partially dried in a circulating air oven at 180° F. Uponexiting from the oven the film was rapidly precipitated by spraying withwater, followed by immersion in a water bath and completely dried in acirculating air oven.

A fourth film was cast as a control but was not subjected to the waterbath so as to obtain conventional non-cellular product.

The resulting level of octabromobiphenyl in the dense film was about 6weight percent. Such a level of flame retardant resulted in an oxygenindex of 32 to 34, depending on thickness and density of the film, ascan be seen above.

The casting conditions and resulting properties are summarized in TableII.

                                      TABLE II                                    __________________________________________________________________________                                            Time In                                         Dry Film                                                                            Tensile                                                                             Propagating                                                                          Dielectric First  Wet Film                            Density                                                                            Thickness                                                                           Modulus                                                                             Tear Strength                                                                        Strength                                                                            Oxygen                                                                             Drying Oven                                                                          Thickness                      Film gm/cc                                                                              mils  psi   gm/mil volts/mil                                                                           Index                                                                              Minutes                                                                              Mils                           __________________________________________________________________________    Roll A                                                                             0.90 5.6   143,000                                                                             14.8   1029  32.9 13.0   18                             Roll B                                                                             1.10 4.5   249,000                                                                             17.2   1400  34.0 17.3   18                             Roll C                                                                             0.83 6.0   134,000                                                                             14.0    965  32.9 10.4   19                             Control                                                                            1.3  2.0   300,000                                                                             8.0    5000  --                                         __________________________________________________________________________

EXAMPLE 2

A casting solution for an aromatic polyamide was prepared by stirring amixture of 16 grams ofpoly(imino-1,3-phenyleneiminocarbonyl-1,3-phenylenecarbonyl), 59 gramsof N,N-dimethyl acetamide and 4.4 grams of lithium chloride which isemployed so as to facilitate solubilizing the polymer. A 25 mil wet filmwas cast onto a glass plate and placed in a circulating air oven for 20minutes and at a temperature of 200° F. so as to partially remove thesolvent. The clear film on the glass plate was then immersed in waterfor one hour and thereafter dried at 350° F. for three hours. A tough,opaque, flexible, non-porous microcellular film having good physical andelectrical properties was produced.

EXAMPLE 3

This Example is illustrative of method 1 described above wherein verylow density, non-porous, microcellular film can be obtained by exposingthe film to water vapor prior to any drying, i.e. the film is notpartially dried before contact with water.

A 15% by weight of polysulfone (from Union Carbide), having thestructure ##STR5## was prepared using N,N-dimethyl formamide. Thesolution was cast onto a glass plate and exposed to humid air until amicrocellular structure was formed as indicated by an opaque off-whitecolor. The film was removed after one hour and dried at 150° C. for onehour. The dried film was non-porous, microcellular, flexible and tough.

EXAMPLE IV

A solution of 10 grams of polyhydantoin (Bayer 4089) was dissolved in 40grams of N,N-dimethyl formamide. A 20 mil thick wet film was cast onto apyrex glass plate and put into an air swept drying oven and held at 250°F. for five minutes. The partially dried clear film was immersed inwater for four (4) hours. The now opaque film was removed from the waterand dried 30 minutes at 250° F., 30 minutes at 350° F. and four hours at400° F. The dried film was opaque, non-porous, microcellular, flexibleand tough.

The microcellular structures of the invention can have incorporatedtherein a wide variety of small particle additives and/or fillers. Theresulting structures are relatively non-brittle compared to a densestructure containing a comparable amount of filler or additive.

Illustrative examples of additives are flame retardants, antioxidants,pigments and the like.

The cellular structures will also be highly useful as separators in fuelcells which do not utilize alkaline electrolytes. The high temperatureresistance, strength and ability to be adhered to conductors such asmetals and carbon as well as the ease of electroplating a strongadherent metal film to the microcellular structures make them uniquelysuitable for many fuel cell and battery component applications.

Furthermore, the ability of the dense and cellular materials describedherein to both adhere tenaciously to metals, carbon graphite, etc.,substrates as well as their high temperature solvent and corrosionresistance to virtually all chemical substances except aprotic solventsand alkalies makes these polymers outstanding material for tank linings,pipe coatings and other thin film protective coatings. Their lowpermeability properties (denser film) are also significant in thisapplication.

Another unique and highly useful application of the microcellular filmcoatings, especially PPA, relies on the unusual low temperatureperformance of these polymers. Thus, coatings for pipes and electricalcable conduit wraps can be used in extremely adverse low temperature,environments as low as about -268° C., with no adverse affect. Thispermits use with liquid helium and liquid nitrogen without losingflexibility and with low dissipation factors.

This permits PPA (cellular film), to be used as insulating andprotective materials in low temperature conductors. Such low temperatureconductors are the clear trend of the future and PPA should play animportant role in these environments.

What is claimed is:
 1. A shaped, relatively low density, essentially gasand liquid impermeable, closed-celled, microcellular polymeric articleof an aromatic polyparabanic acid, obtained by:(a) forming a solution ofthe polymeric material in a dipolar, aprotic solvent for said material;(b) casting said polymer solution onto a suitable surface to form anintermediate stage structure or extruding a fiber; (c) partially dryingthe casted or extruded polymer solution; (d) exposing said resultingstructure or fiber to a non-solvent thereby precipitating said polymerin the presence of said non-solvent; (e) drying said precipitated solid;and (f) recovering said shaped microcellular article.
 2. The articleaccording to claim 1 wherein said non-solvent is water.
 3. The articleaccording to claim 1 wherein said solvent is dimethylformamide.
 4. Ashaped, relatively low density essentially gas and liquid impermeable,closed-celled, microcellular polymeric article of an aromaticpolyparabanic acid, obtained by:(a) forming a solution of the polymericmaterial in a dipolar, aprotic solvent for said material; (b) castingsaid polymer solution onto a suitable surface to form an intermediatestage structure or extruding a fiber; (c) exposing said resultingstructure or fiber to a non-solvent atmosphere of high humidity therebyprecipitating solid polymer in the presence of said non-solventatmosphere; (d) drying said precipitated solvent; and (e) recoveringsaid shaped microcellular article.
 5. The article according to claim 4wherein the non-solvent is water and the solvent is dimethylformamide.6. An article according to claim 1 in the shape of a film.
 7. Thearticle according to claim 1 in the shape of a fiber.
 8. An articleaccording to claim 1 which has a density of about 0.3 to 1.5 g/cm³. 9.An article according to claim 1 having a density of about 0.3 to about1.2 g/cm³.
 10. An article according to claim 1 being from about 1 mil toabout 20 mils in thickness.
 11. An article according to claim 4 in theshape of a film.
 12. The article according to claim 4 in the shape of afiber.
 13. An article according to claim 4 which has a density of about0.3 to 1.5 g/cm³.
 14. An article according to claim 13 having a densityof about 0.3 to about 1.2 g/cm³.
 15. An article according to claim 4being from about 1 mil to about 20 mils in thickness.